Cyclic introduction, in which two types of material gases are caused to flow alternately in a chamber, is beginning to be developed as a method for forming a thin film utilizing a low pressure vapor deposition apparatus. In the case of cyclic introduction, the amount of the material gas consumed in the chamber is exceedingly low. Therefore, nearly all of the material gas that is introduced is exhausted from the chamber. The material gas is exhausted from the chamber by using a pump such as a dry vacuum pump, etc. When the material gas flows into the pump, by-products that are caused by the material gas are produced inside the pump. There is a possibility that the by-products may cause a decrease of the pump life by adhering to the pump, plugging the exhaust pipes, etc. It is desirable to increase the pump life.